1. Field of the Invention
This invention relates to an improved ram bore profile for a variable bore packer ram used in a ram-type blowout preventer used for oil and gas drilling operations. The improved ram bore profile allows drill pipe within a given range of sizes to be supported or "hung off" with increased load capacity in comparison to existing designs. Additionally, the current invention ensures the drill pipe weight is in fact supported on the ram body and not on the variable bore packer as in prior art designs. This ensures the variable bore packer is not subjected to unbalanced loads and thereby improves the service life of the packer. Ram-type blowout preventers are part of a pressure control system used in oil and gas drilling operations to control unexpected well bore pressure spikes or "kicks" as they are commonly referred to in the industry.
The blowout preventer has a body with a vertical bore and a pair of laterally disposed opposing bonnet assemblies. Each bonnet assembly includes a piston which is laterally moveable within the bonnet assembly by pressurized hydraulic fluid. Replaceable sealing elements called "packers" are mounted on the ends of the pistons which extend into the blowout preventer bore. When these pistons are moved to a closed position, commonly referred to as "closing the blowout preventer" or "closing the rams", the vertical bore of the blowout preventer is sealed and the "kick" is contained. These "packers" are available in a variety of configurations designed to seal the blowout preventer bore when the opposing pistons are moved to their closed position.
One type of packer has ends designed to seal around pipe of a specific size in the blowout preventer bore when the blowout preventer is "closed." Other packers are configured to seal around a range of pipe sizes. It is the type designed to seal around a range of pipe sizes, called variable bore ram packers, and the ram body in which they are mounted to which the present invention is directed. The ram packers form a pressure tight seal during a kick until the well bore pressure can be controlled. The well bore pressure can reach several thousand pounds per square inch during a "kick." Each ram packer has a semicircular opening in its front face to form a seal around 180.degree. of the outer periphery of the pipe. When the rams are closed as described above, the opposing ram packers meet and seal the entire 360.degree. periphery of the pipe.
During drilling operations, it is sometimes necessary to support the weight of the joints of drill pipe or "drill string" as it is commonly known on the rams of the blowout preventer. This is done by dosing the rams around the straight diameter of the drill pipe and the allowing the drill string to slide downward until an enlarged diameter end, called a tool joint, contacts the top of the ram. Each section or joint of drill pipe has a tool joint formed at each end and the tool joints at opposite ends of a given drill pipe section are threaded with mating male and female threads, respectively. The transition between the straight diameter of the drill pipe and the larger diameter of the tool joint is a tapered shoulder, often referred to as an elevator shoulder. This tapered elevator shoulder contacts the top of the ram and the weight of the drill string is supported by this contact.
Prior art variable bore ram designs typically used a semi-circular cutout in the face of ram body. This semicircular cutout had to be sized to accommodate the largest diameter drill pipe that the variable bore ram packer was designed to seal around. This often resulted in little or no contact between the ram body and the tool joint when a smaller diameter drill pipe and its tool joint were used. In this case, when the rams were closed and the drill string lowered into contact with the ram, it was the portion of the variable bore ram packer protruding beyond the face of the ram that supported the weight of the drill string. The variable bore ram packers were not designed or intended for this type of service. As a result, sometimes the drill string was dropped when the variable bore ram packer could not longer support the weight of the drill string. At the very least, the service life of the variable bore packer was greatly compromised when the packer supported the weight of the drill string. The improved ram bore profile for a variable bore packer ram of the current invention offers a substantial improvement by offering a variable bore packer ram body that can support the weight of the drill string for a range of drill pipe sizes.
2. Description of Related Art
U.S. Pat. No. 4,089,532 to A. I. Kamyshnikov shows a blowout preventer ram assembly with a variable bore ram packer.
The blowout preventer shown in U.S. Pat. No. 5,013,005 to J. G. Nance shows a packer capable of sealing around two strings of tubing disposed in the blowout preventer bore.